Because of excellent weathering resistance, ozone resistance and heat resistance of ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubbers, they have been broadly used for automobile parts, building materials, electrical wires and industrial parts. In such uses, the ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubbers are conventionally kneaded, like other rubbers, with various compounding ingredients such as reinforcing agents (e.g., carbon black), fillers (e.g., talc, clay), softeners, vulcanization accelerators and vulcanizing agents by means of batch type kneading machines such as Banbury mixer or roll, or each kind of extruders to prepare compounded rubbers.
In the preparation of products of the ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubbers, polymers each of which has different molecular weight and molecular weight distribution are combined with various compounding ingredients to adjust balance between processability and product property.
The compounded rubbers are generally designed to have their respective compositions according to the hardness of the resulting rubber products. In order to obtain a certain harness, carbon black, talc, clay, silica, calcium carbonate, etc. are used in consideration of balance between cost and product property. The reinforcing agents and the fillers mentioned above exert effects of increasing a product hardness in greater or lesser degrees, but they have nature of decreasing processability of the resulting compounded rubbers because of a rise in the viscosity of the compounded rubbers. Additionally, the reinforcing agents and the fillers also have nature of decreasing extensibility that is an important property of the rubber products.
Other than the reinforcing agents and fillers having such nature as mentioned above, there are known polyolefin resins (e.g., polyethylene) which are used as fillers exerting an effect of increasing a product hardness to almost the same degrees as those carbon black and exhibit an effect of decreasing a viscosity of the compounded rubbers thereby to improve the processability. Polyethylene is practically used in compounding with rubbers.
On the other hand, with respect to polypropylene if the amount thereof is not more than 50 parts by weight based on 100 parts by weight of the total amount of EPDM and polypropylene, the resulting compounded rubber takes a "sea-island structure" wherein the EPDM phase is like a sea and the polypropylene phase is like an island. That is, polypropylene is not blended with EPDM, unlike polyethylene, and therefore polypropylene cannot be usually used as a reinforcing agent for vulcanized rubbers.
Even if a polypropylene/EPDM blend, wherein polypropylene having a mean particle diameter of not more than 2 .mu.m is microdispersed, is prepared by a process comprising the step of melt blending EPDM with polypropylene at a temperature of not lower than the melting point of polypropylene, when employing a step of roll processing, by means of a conventional kneading machine such as a Banbury mixer, which comprises adding carbon black, an oil and a filler to the polypropylene/EPDM blend to prepare a compounded rubber and then adding a vulcanization accelerator and a vulcanizing agent to the compounded rubber with cooling the compounded rubber, as the compounded rubber kept at the kneading temperature is cooled in the roll processing stage, adhesion property of the compounded rubber to the rolls becomes poor. Especially when employing steps which comprise melt blending polypropylene with EPDM at a temperature of not lower than the melting point of polypropylene to obtain a polypropylene/EPDM blend and immediately cooling the polypropylene/EPDM blend with rolls, the compounded rubber does not sufficiently adhere to the rolls, and finally the roll processing becomes impossible. In that case, the Mooney viscosity of the compounded rubber rises too high, whereby a molding process such as extrusion molding, transfer molding or injection molding may become difficult. Further, when the blend is vulcanized, the product hardness, that is a most important property thereof, also rises too high, whereby vulcanized rubber products of the aimed product hardness cannot be obtained.
Polyolefin resins, such as polyethylene and polypropylene, are generally kneaded by kneading machines such as a Banbury mixer, an intermixer and a kneader. In the kneading procedure, the temperature of the kneading machines needs to be raised to not lower than the melting point of the polyolefin resins.
However, these kneading machines are originally designed so that the kneading temperature hardly rises for the purpose of inhibiting burning marks of the compounded rubber during the kneading procedure. Therefore, it was difficult to sufficiently disperse and knead the polyolefin resin compounded, and as a result a problem of poor dispersion of polyolefin takes place. For example, when EPR or EPDM is used for automobile parts after vulcanized, it is sometimes blended with a polyethylene resin, together with a reinforcing agent such as carbon black, to adjust the product hardness. In this blending using a kneading machine such as a Banbury mixer or an intermixer, the polyethylene resin is dispersed insufficiently to cause remaining of agglomerates of the polyethylene resin in the resulting product, whereby bad appearance of the product or lowering of the product property takes place.
The polyethylene resin is generally handled in the form of pellets, and therefore it is unsuitable for the conventional rubber processing equipment provided on the premise of veil handling, for example, some pellets of the polyethylene resin are caught in gaps in the kneading machine and the pellets having been not kneaded are discharged from the kneading machine, whereby bad appearance of the product or lowering of the product property may occur. Accordingly, an improvement in the kneading compatibility of the polyethylene resin pellets with rubbers is desired.
By the way, compositions having ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubbers are conventionally known as vulcanizable foaming rubber compositions. Foamed products of the compositions are widely used as sealing materials for automobiles, buildings and electrical appliances. Because of their excellent heat resistance and thermal aging resistance, the foamed products are favorably used as long-life sealing materials.
However, the compositions having the conventional ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubbers have low flowability, so that it is difficult to produce large-sized foamed-in place sponges from the compositions.
Accordingly, various studies to produce large-sized foamed-in-place sponges (foamed products) have been made. For example, there is known a method of using a plasticizer in a large amount to improve flowability of the ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubber compositions. In this method, however, it is difficult to obtain foamed products of satisfactory strength and modulus, and therefore practically useful large-sized foamed-in-place sponge products can be hardly obtained. A method of using an ethylene/.alpha.-olefin/nonconjugated polyene copolymer rubber of high ethylene content has been proposed. In this method, however, sealing properties of the foamed products become insufficient, though the strength of the modulus are improved.
Consequently, now desired is development of a vulcanizable ethylene copolymer rubber composition by the use of which the polyolefin resin can be reliably and sufficiently dispersed for a short period of time by means of a rubber kneading machine conventionally used in the rubber industry, thereby to easily supply rubber products of high and stable quality at a low cost, and development of a process for preparing the rubber composition is also desired. Further, development of a rubber composition capable of producing large-sized foamed products well-balanced balanced among the strength, modulus and sealing properties is also desired.
The present invention is intended to solve such problems associated with the prior art as described above, and it is a main object of the invention to provide a vulcanizable ethylene copolymer rubber composition by the use of which the polyolefin resin can be reliably and sufficiently dispersed for a short period of time by means of a rubber kneading machine conventionally used in the rubber industry, thereby to easily supply rubber products of high and stable quality at a low cost. It is another object of the invention to provide a process for preparing the rubber composition.